This invention relates to shingling and stacking of conveyed sheet material with the incorporation of a system for controlling sheet feed. The invention is an improvement over the above-identified U.S. Pat. No. 4,200,276, the entire contents of which is incorporated herein by reference.
In U.S. Pat. No. 4,200,276, hereinafter referred to as the prior patent, sheets of corrugated paperboard or the like are cut and fed in line in succession from an input (corrugator) conveyor section, through a speed-up conveyor section and hence to a vacuum conveyor section where the sheets are shingled. The shingled sheets are then fed through an accumulating conveyor section and a stack infeed conveyor section to a sheet stacker. The patent discloses numerous controls, including a system control circuit (FIG. 8) for controlling variable speed conveyor motors and other apparatus. The motors, including a shingling conveyor motor, are initially pre-set for a "normal" speed, and, except for the speed-up conveyor motor, are then varied from normal by the controls in response to movement of the sheets along the device.
Basically, the speed-up conveyor of the patented device increases the speed of the corrugated sheets over the speed at the input conveyor section during the entire operation. The normal pre-set speed of the other conveyors downstream of the speed-up conveyor is usually substantially less than that of the input conveyor, and generally the same for all downstream conveyors.
In operation of the patented device, the downstream conveyors are all speeded up to a generally similar speed, during which time the sheets are shingled into stacks which are ultimately separated. The conveyors are then individually and successively slowed in a downstream direction to cause separate shingled stacks to pull away from each other. Once a shingled stack has been fully discharged into the stacker, the downstream conveyors are returned to normal speed. The process repeats itself for each group of sheets, depending upon how many sheets the stacker can handle at one time.
Heretofore, the pre-set normal speed of the vacuum shingling conveyor (and other downstream conveyors) has been substantially lower than the input speed, such as 25% thereof. At moderate sheet input speeds (such as 500 ft./min.) and long individual sheet lengths (such as 200 inches), no essential problems have arisen with the vacuum shingling conveyor. However, it has been noted that as sheet input speeds are increased (such as to 1,000 ft./min.) and/or individual sheets are shortened (such as to 30 inches), optimum shingling has not taken place; that is, the sheets have not formed into a neat stack but have skewed and slid in a longitudinal direction in an overrunning action.
It is believed that the problem of "scattered" shingles is due to the reduction in the size of the tail on each successive sheet being shingled due to the abrupt change of sheet speed as it enters the shingler, accompanied especially by relatively high overall speeds. The vacuum box on the shingler cannot firmly hold high speed and/or small tail sheets in place. Merely increasing the pre-set normal speed of the vacuum shingling conveyor (and other downstream conveyors) to, for example, 50% of the input speed to solve the problem, may overrun the capacity of the stacking device because sheets will be delivered to it faster.
It is an aim of the invention to substantially reduce or eliminate the problem of shingle scattering at the vacuum conveying section. It is a further aim of the invention to solve the problem, even with high input speeds and small sheet tails. It is yet another aim to solve the problem without overrunning the capacity of the sheet stacking device.
In accordance with the various aspects of the invention, the shingling section of the device of U.S. Pat. No. 4,200,276 is provided with a combination of the usual vacuum shingler together with a pre-shingling means such as a second shingler disposed just upstream of the usual or first shingler. The second shingler is disposed at the discharge of the speed-up conveyor. A setting control is provided to pre-set the pre-shingler conveyor speed in correlation with the input conveyor speed and the length of individual sheets. The pre-set pre-shingling conveyor speed remains constant during operation of the device and is set at a speed higher than the normal pre-set speed of the first or usual vacuum shingling conveyor, which is controlled, as before, by the system circuit.
The higher speed of the auxiliary or pre-shingling vacuum conveyor provides for less of a shock to the sheets (given the same input sheet speed) than a lower speed would and may be calculated to prevent scattered shingles. At the same time, the first or main shingling conveyor, being set at a lower speed than that of the second or pre-shingling conveyor, receives and re-shingles the sheets and passes them on down the line, with the other downstream conveyors functioning exactly as in the prior patent. The result is that the stacker may receive the same number of sheets per unit of time as without the pre-shingling device, but the shingled stacks are no longer skewed or the like.
Since the first vacuum shingling conveyor and the second or vacuum pre-shingling conveyor are both dependent on the input conveyor speed, the first conveyor is correlated with and bears a known relationship to the second conveyor. Thus, the first conveyor and the conveyors downstream thereof travel at a speed during operation which is effectively a percentage of the second conveyor speed.